253
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled by setting
the DAC Auto Trigger Enable bit, DAATE in DACON. The trigger source is selected by setting the DAC Trigger
Select bits, DATS in DACON (See description of the DATS bits for a list of the trigger sources). When a positive
edge occurs on the selected trigger signal, the DAC converts the value present on the DACH and DACL registers
in accordance with the register DACON setting. This provides a method of starting conversions at fixed intervals. If
the trigger signal is still set when the conversion completes, a new conversion will not be started. If another positive
edge occurs on the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be set
even if the specific interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can
thus be triggered without causing an interrupt. However, the interrupt flag must be cleared in order to trigger a new
conversion at the next interrupt event.
24.4.1
DAC voltage reference
The reference voltage for the ADC (V
REF) indicates the conversion range for the DAC. VREF can be selected as
either AV
CC, internal 2.56V reference, or external AREF pin.
AV
CC is connected to the DAC through a passive switch. The internal 2.56V reference is generated from the inter-
nal bandgap reference (V
BG) through an internal amplifier. In either case, the external AREF pin is directly
connected to the DAC, and the reference voltage can be made more immune to noise by connecting a capacitor
between the AREF pin and ground. V
REF can also be measured at the AREF pin with a high impedance voltmeter.
Note that V
REF is a high impedance source, and only a capacitive load should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference voltage
options in the application, as they will be shorted to the external voltage. If no external voltage is applied to the
AREF pin, the user may switch between AV
CC and 2.56V as reference selection. The first DAC conversion result
after switching reference voltage source may be inaccurate, and the user is advised to discard this result.
24.5
Register description
The DAC is controlled via three dedicated registers, the DACON register which is used for DAC configuration, and
the DACH and DACL which are used to set the value to be converted.
24.5.1
DACON – Digital to Analog Conversion Control Register
Bit 7 – DAATE: DAC Auto Trigger Enable bit
Set this bit to update the DAC input value on the positive edge of the trigger signal selected with the DACTS2-0 bit
in DACON register.
Clear it to automatically update the DAC input when a value is written on DACH register.
Bit 6:4 – DATS[2:0]: DAC Trigger Selection bits
These bits are only necessary in case the DAC works in auto trigger mode. It means if DAATE bit is set.
Bit
7
654
3
2
1
0
DAATE
DATS2
DATS1
DATS0
-
DALA
DAOE
DAEN
DACON
Read/write
R/W
-
R/W
Initial value
0